Affinity Labeling of Lymphocyte Antigen Receptors. The technique of affinity labeling offers the possibility of radiolabeling specific cell surface receptors on the basis of their binding specificity without concern for other structural characteristics of the receptor. Lymphocytes, even from hyperimmune animals, are too heterogenous with respect to receptor specificity to be successfully affinity labeled. The murine myeloma, MOPC 315, provides a useful model for analysing the extent of homogeneity required. If these cells are manipulated to yield a population where 70% or more of the cells exhibit a high density of DNP-specific receptors on their surface, azide analogs of DNP will affinity label a molecule similar in molecular weight to mouse alpha chain. Anti-IgA can be used to serologically precipitate up to 40% of the TCA precipitable counts from cells so labeled. Non-specific labeling with DNP analogs is due, in great part, to their hydrophobic nature and suggests that other antigens and their analogs may be more useful. Selection of lymphocytes with a single receptor specificity on hapten-gelatin conjugates is being utilized to produce cell populations comparable to the myeloma model. It appears that given a cell population sufficiently homogeneous with respect to the specificity of its antigen receptors, affinity labeling can be use to aid in the identification of such receptors.